Ok, so it seems there is no way to hook into the garbage collection of memcache, in order to clean out the entry in the database. This is a pitty, as it now means I do need to manage this inside the database, while I hoped I could rely on memcache for this, where I will need to write to anyways when ever I change data inside the session. Doing this update every X minutes makes sense in that case.

I am not yet decided on what table type to use on the MySQL slaves. If I were to use MyISAM, I would get instantaneous COUNT(*) and I would save some memory I could rather allocate to memcache. A HEAP table would generally be all around faster for the writes, though since the table will only consist of fixed length columns, even updates should be decently fast with MyISAM. Anyways, this should be easy to test out later.

@Ivo/Robin: The way you describe the Zend session cluster it seems to make most sense if you do DNS based load balancing, where you expect the user to hit the same server for a number of requests. Having a simple switch to either use memory or disc storage is also nice. But I do not see how this is any better than memcache if you are not DNS load balancing.

There's always so much talk about memcache, and sure it can fix some problems, but why don't we hear more about MCache?

http://www.mohawksoft.org/?q=node/8

MCache has a PHP session handler and an API for ... well, caching.